The present invention relates generally to agricultural harvesting implements and more particularly relates to ground sensing mechanisms utilized on row crop harvesting units to raise and lower the unit in accordance with variations in the ground surface over which the units are traveling.
It has heretofore been known to employ hydraulic height sensing systems for raising and lowering cantilevered row harvesting units, see for example U.S. Pat. No. 3,417,553 to A. L. Hubbard, U.S. Pat. No. 3,643,407 to Hubbard et al and U.S. Pat. No. 3,714,767 to Hubbard et al. Typically these sensing mechanisms employ a ground-engaging shoe or feeler supported on the harvesting unit and mechanically coupled with a control valve which continuously and adjustably varies the flow of hydraulic fluid to a hydraulic lift cylinder for raising and lowering the harvesting unit in accordance with variations in the ground surface over which the harvesting unit travels.
Agricultural harvesters commonly utilizing such a height sensing mechanism typically have two or more row crop harvesting units supported on the harvester frame in cantilevered fashion. Each harvesting unit is provided with its own ground-engaging shoe, lift cylinder and hydraulic control valve for adjustably varying the height of the row crop unit. It has further been typical to provide a common hydraulic fluid system for both the steering and height sensing functions on cotton harvesters such as cotton strippers or cotton pickers.
The hydraulic system employed on such harvesters has been an open center system due to the limited number of hydraulic functions required on such a harvester. Accordingly, both the steering and height sensing functions were provided by a single pump and a flow divider was utilized between the two systems to assure adequate steering and height sensing hydraulic flow capabilities. Until recently, cotton pickers and cotton stripper harvesters were provided with no more than two row crop harvesting units. However four row unit harvesters have recently been introduced. With the introduction of these harvesters a parallel hydraulic arrangement was required whereby each individual row unit could be vertically adjusted in accordance with its particular ground surface variation. Accordingly, a second flow divider was introduced into the height sensing hydraulic system to assure the proper hydraulic flow to each unit's lift cylinder.
The recent introduction of these four unit harvesters requiring individual sensing capacities has presented a problem not incurred with two unit harvesters of the past. In the southwest United States, cotton is often planted in adjacent rows with the third row being skipped to provide better irrigation and productivity. To harvest such fields with a four row harvester, one unit was raised to its full up position. This required that one lift cylinder be extended for an extended period of time. Since an open center hydraulic system is utilized, the hydraulic line pressure would increase and the relief valve provided to protect the hydraulic system would open returning the fluid to the reservoir. Because the pressure at which the relief valve opened was significantly higher than the operating pressure required to assure adequate pressure reserve for the steering system, the relief valve would not open until the line pressure had risen sufficiently to increase significantly the fluid temperature. Consequently, the elevation of the row unit for just a short period of time caused the hydraulic fluid temperature to be increased enough to cause the pump rings and pump to fail, thereby resulting in the loss of the height sensing system as well as the steering system for the harvester.
Further complicating the conversion to a four row header unit and the utilization of an open center height sensing unit mechanism was an additional problem. As a particular row unit was elevated during harvesting of skip-row cotton or as a result of some foreign object jamming the height sensing ground-engaging shoe the corresponding pressure increase would result in a slight jump in the other three parallel lift cylinders even though the flow dividers assured a relatively constant flow. This resulted in less efficient harvesting of the cotton.